The present invention relates to a heat sink capable of managing the heat from a heat source such as an electronic device.
With the development of more and more sophisticated electronic devices, including those capable of increasing processing speeds and higher frequencies, having smaller size and more complicated power requirements, and exhibiting other technological advances, such as microprocessors and integrated circuits in electronic and electrical components and systems as well as in other devices such as high power optical devices, relatively extreme temperatures can be generated. However, microprocessors, integrated circuits and other sophisticated electronic components typically operate efficiently only under a certain range of threshold temperatures. The excessive heat generated during operation of these components can not only harm their own performance, but can also degrade the performance and reliability of the overall system and can even cause system failure. The increasingly wide range of environmental conditions, including temperature extremes, in which electronic systems are expected to operate, exacerbates the negative effects of excessive heat.
With the increased need for heat dissipation from microelectronic devices, thermal management becomes an increasingly important element of the design of electronic products. Both performance reliability and life expectancy of electronic equipment are inversely related to the component temperature of the equipment. For instance, a reduction in the operating temperature of a device such as a typical silicon semiconductor can correspond to an increase in the processing speed, reliability and life expectancy of the device. Therefore, to maximize the life-span and reliability of a component, controlling the device operating temperature within the limits set by the designers is of paramount importance.
Several types of heat dissipating components are utilized to facilitate heat dissipation from electronic devices. The present invention is directly applicable to finned heat sinks.
These heat sinks facilitate heat dissipation from the surface of a heat source, such as a heat-generating electronic device, to a cooler environment, usually air. The heat sink seeks to increase the heat transfer efficiency between the electronic device and the ambient air primarily by increasing the surface area that is in direct contact with the air or other heat transfer media. This allows more heat to be dissipated and thus lowers the electronic device operating temperature. The primary purpose of a heat dissipating component is to help maintain the device temperature below the maximum allowable temperature specified by its designer/manufacturer.
Typically, the heat sinks are formed of a metal, especially copper or aluminum, due to the ability of metals like copper to readily absorb heat and transfer it about its entire structure. Copper heat sinks are often formed with fins or other structures to increase the surface area of the heat sink, with air being forced across or through the fins (such as by a fan) to effect heat dissipation from the electronic component, through the copper heat sink and then to the air.
The use of copper or aluminum heat dissipating elements can present a problem because of the weight of the metal, particularly when the heat transmitting area of the heat dissipating component is significantly larger than that of the electronic device. For instance, pure copper weighs 8.96 grams per cubic centimeter (g/cm3) and pure aluminum weighs 2.70 g/cm3.
For example, in many applications, several heat sinks need to be arrayed on, e.g., a circuit board to dissipate heat from a variety of components on the board. If metallic heat sinks are employed, the sheer weight of the metal on the board can increase the chances of the board cracking or of other equally undesirable effects, and increases the weight of the component itself. For portable electronic devices, any method to reduce weight while maintaining heat dissipation characteristics is especially desirable.
Another group of materials suitable for use in heat sinks are those materials generally known as graphites, but in particular graphites such as those based on natural graphites and flexible graphite as described below. These materials have a high thermal conductivity in the plane of the sheet of material of 370 W/mxc2x0 K. This is higher than either aluminum (200 W/mxc2x0 K) or copper (360 W/mxc2x0 K). Also, the graphite materials are much lighter in weight and thus provide many advantages over copper or aluminum.
Graphites are made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as graphene layers or basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphites consist of crystallites of considerable size: the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. It should be noted that graphites possess anisotropic structures and thus exhibit or possess many properties that are highly directional e.g. thermal and electrical conductivity and fluid diffusion.
Briefly, graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two axes or directions are usually noted, to wit, the xe2x80x9ccxe2x80x9d axis or direction and the xe2x80x9caxe2x80x9d axes or directions. For simplicity, the xe2x80x9ccxe2x80x9d axis or direction may be considered as the direction perpendicular to the carbon layers. The xe2x80x9caxe2x80x9d axes or directions may be considered as the directions parallel to the carbon layers or the directions perpendicular to the xe2x80x9ccxe2x80x9d direction. The graphites suitable for manufacturing flexible graphite sheets possess a very high degree of orientation.
As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Natural graphites can be treated so that the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the xe2x80x9ccxe2x80x9d direction, and thus form an expanded or intumesced graphite structure in which the laminar character of the carbon layers is substantially retained.
Graphite flake which has been greatly expanded and more particularly expanded so as to have a final thickness or xe2x80x9ccxe2x80x9d direction dimension which is as much as about 80 or more times the original xe2x80x9ccxe2x80x9d direction dimension can be formed without the use of a binder into cohesive or integrated sheets of expanded graphite, e.g. webs, papers, strips, tapes, foils, mats or the like (typically referred to as xe2x80x9cflexible graphitexe2x80x9d). The formation of graphite particles which have been expanded to have a final thickness or xe2x80x9ccxe2x80x9d dimension which is as much as about 80 times or more the original xe2x80x9ccxe2x80x9d direction dimension into integrated flexible sheets by compression, without the use of any binding material, is believed to be possible due to the mechanical interlocking, or cohesion, which is achieved between the voluminously expanded graphite particles.
In addition to flexibility, the sheet material, as noted above, has also been found to possess a high degree of anisotropy with respect to thermal and electrical conductivity and fluid diffusion, comparable to the natural graphite starting material due to orientation of the expanded graphite particles and graphite layers substantially parallel to the opposed faces of the sheet resulting from very high compression, e.g. roll pressing. Sheet material thus produced has excellent flexibility, good strength and a very high degree of orientation.
Briefly, the process of producing flexible, binderless anisotropic graphite sheet material, e.g. web, paper, strip, tape, foil, mat, or the like, comprises compressing or compacting under a predetermined load and in the absence of a binder, expanded graphite particles which have a xe2x80x9ccxe2x80x9d direction dimension which is as much as about 80 or more times that of the original particles so as to form a substantially flat, flexible, integrated graphite sheet. The expanded graphite particles that generally are worm-like or vermiform in appearance, once compressed, will maintain the compression set and alignment with the opposed major surfaces of the sheet. The density and thickness of the sheet material can be varied by controlling the degree of compression. The density of the sheet material can be within the range of from about 0.04 g/cm3 to about 2.0 g/cm3. The flexible graphite sheet material exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet, with the degree of anisotropy increasing upon roll pressing of the sheet material to increase orientation. In roll pressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the opposed, parallel sheet surfaces comprises the xe2x80x9ccxe2x80x9d direction and the directions ranging along the length and width, i.e. along or parallel to the opposed, major surfaces comprises the xe2x80x9caxe2x80x9d directions and the thermal, electrical and fluid diffusion properties of the sheet are very different, by orders of magnitude, for the xe2x80x9ccxe2x80x9d and xe2x80x9caxe2x80x9d directions.
There is a continuing need for improved heat sink designs which provide relatively low thermal resistance and relatively low weight as compared to prior designs.
The present invention provides a finned heat sink constructed from anisotropic graphite material, wherein the heat sink is made from alternating longer and shorter sheets of graphite material, which sheets are sandwiched together such that the longer sheets extend beyond the shorter sheets to define the fins of the heat sink apparatus.
Preferably, the longer and shorter sheets have their base ends aligned together to define a generally planar base surface for engaging an electronic device to be cooled by the heat sink apparatus.
In a preferred embodiment of the invention, the longer and shorter sheets are each constructed from laminated resin-impregnated graphite sheet material, and the resin of the resin-impregnated material provides sufficient adhesive characteristics to bond the alternating longer and shorter sheets together to form the heat sink apparatus.
Also, a copper bar may be inserted in the base, transverse to the plane of the sheets, to spread heat from the heat source to all of the fins.
A thermal management apparatus includes a heat sink like that just described in conjunction with an electronic heat source having a heat transfer surface defined thereon and in heat transfer engagement with the planar base surface of the heat sink apparatus.
In another aspect of the invention a method of manufacturing such a heat sink apparatus is provided, which method includes the steps of:
(a) forming a plurality of shorter and longer pieces of graphite material;
(b) assembling the shorter and longer pieces so that base ends of the pieces are aligned together to define a generally planar base surface, and so that the longer pieces extend beyond the shorter pieces to define fins extending in a direction away from the base surface; and
(c) applying pressure to the pieces in a direction across a thickness of the pieces to bond the pieces together.
Accordingly, it is an object of the present invention to provide an improved heat sink design for thermal management of electronic devices.
And another object of the present invention is the provision of a finned heat sink apparatus constructed from alternating longer and shorter sheets of graphite material which are sandwiched together so that the longer sheets extend beyond the shorter sheets to define fins of the heat sink apparatus.
And another object of the present invention is the provision of a heat sink apparatus formed of graphite sheets all having their directions of higher thermal conductivity directed substantially perpendicularly away from a base surface of the heat sink apparatus so as to rapidly direct heat energy away from the device being cooled.
Other and further objects, features, and advantages of the present invention will be readily apparent to those skilled in the art, upon a reading of the following disclosure when taken in conjunction with the accompanying drawings.